Crosslinked polyvinyl amine copolymer gels for use under harsh reservoir conditions

ABSTRACT

A gel-forming composition capable of plugging highly permeable zones in a subterranean formation. The composition comprises water, a viscosifying amount of a water-dispersible polyvinyl amine copolymer and a crosslinking agent which is a mixture of an aldehyde and a phenolic component in an amount effective to cause gelation of the aqueous solution of the copolymer. The resultant gel is exceptionally thermally stable and, therefore, can be used as an effective profile control agent in all enhanced oil recovery operations, including steam flooding.

FIELD OF THE INVENTION

This invention relates to organically crosslinked polyvinyl aminecopolymer gels and to the use thereof in controlling the permeability ofsubterranean oil-bearing formations.

BACKGROUND OF THE INVENTION

Generally, in the production of oil from subterranean formations, only asmall fraction of the total oil present within the formation can berecovered through the use of primary recovery methods which utilize onlythe natural forces present in the reservoir. To recover additional oil,a variety of supplemental production techniques have been developed. Inthese supplemental techniques, commonly referred to as secondary ortertiary recovery operations, a fluid is introduced into the oil-bearingformation in order to displace oil to a production system comprising oneor more production wells. The displacing or "drive" fluid may be anaqueous liquid such as brine of fresh water, a gas such as carbondioxide, steam or dense-phase carbon dioxide, an oil-miscible liquidsuch as butane, or an oil and water-miscible liquid such as an alcohol.Often, the most cost-effective and desirable secondary recovery methodsinvolve the injection of steam. In practice, a number of injection andproduction wells will be used in a given field arranged in conventionalpatterns such as a line drive, a five spot or inverted five spot, or aseven spot or inverted seven spot.

In the use of the various flooding techniques, it has become a commonexpedient to add various polymeric thickening agents to the drive fluidto increase its viscosity to a point where it approaches that of the oilto be displaced, thus improving the displacement of oil from theformation. The polymers used for this purpose are often said to be usedfor "mobility" control.

Another problem encountered is that cerain injected drive fluids may bemuch lighter than the reservoir fluids and thus separate by gravity,rising toward the top of the flowing region and resulting in thebypassing of the lower regions. This phenomena is known as gravityoverride.

Also encountered in the use of the various flooding techniques is asituation caused by the fact that different regions or strata often havedifferent permeabilities. When this situation is encountered, the drivefluid may preferentially enter regions of higher permeability due totheir lower resistance to flow rather than the regions of lowpermeability where significant volumes of oil often reside.

It therefore is often desirable to plug the regions of highpermeability, or "thief" zones, either partly or entirely, so as todivert the drive fluid into regions of lower permeability. Themechanical isolation of these thief zones has been tried but verticalcommunicating among reservoir strata often renders this methodineffective. Physical plugging of the high permeability regions bycements and solid slurries has also been tried with varying degrees ofsuccess; however, these techniques have the drawback thatstill-productive sites may be permanently closed.

As a result of these earlier efforts, the desirability of designing aslug capable of sealing off the most permeable layers so that the drivefluid would be diverted to the underswept, "tighter" regions of thereservoir, became evident. This led to the use of oil/water emulsions,as well as gels and polymers for controlling the permeability of theformations. This process is frequently referred to as "floodconformance" or "profile control", a reference to the control of thevertical permeability profile of the reservoir. Profile control agentswhich have been proposed include oil/water emulsions, gels, e.g.,lignosulfate gels and polymeric gels, with polymeric gels being the mostextensively applied in recent years.

Among the polymers so far examined for improving flood conformance arepolyacrylamides, polysaccharides, celluloses, furfural-alcohol andacrylic/epoxy resins, silicates and polyisocyanurates. A major part ofthis work has been conducted with the polyacrylamides, both in theirnormal, non-crosslinked form, as well as in the form of metal complexes,as described, for example, in U.S. Pat. Nos. 4,009,755, 4,069,869 and4,413,680. In either form, the benefical effects derived from thesepolyacrylamides seem to dissipate rapidly due to shear degradationduring injection and sensitivity to reservoir brines, high pH and hightemperature. To overcome these problems and to achieve deeper polymerpenetration into the reservoir, dilute solutions of these polymers havesometimes been injected first and then complexed in situ.

Another group of polymeric thickeners which has received considerableattention for use in improving flooding are polysaccharides,particularly those produced by the action of bacteria of the genusXanthomonas on carbohydrates. For example, U.S. Pat. Nos. 3,757,863 and3,383,307 disclose a process for mobility control by the use ofpolysaccharides.

U.S. Pat. Nos. 3,741,307, 4,009,755 and 4,069,869 disclose the use ofpolysaccharides in the control of reservoir permeability. U.S. Pat. No.4,413,680 describes the use of crosslinked polysaccharides for selectivepermeability control in oil reservoirs.

U.S. Pat. No. 3,908,760 describes a polymer waterflooding process inwhich a gelled, water-soluble Xanthomonas polysaccharide is injectedinto a stratified reservoir to form a slug, band or front of gelextending vertically across both high permeability and low permeabilitystrata. This patent also suggests the use of complexed polysaccharidesto block natural or man-made fractures in formations. The use ofpolyvalent metal ions for cross-linking polysaccharides is alsodisclosed in U.S. Pat. No. 3,810,822.

Another type of polysaccharide which has been experimented with in thearea of profile control is the non-xanthan, heteropolysaccharide S-130.S-130 is a member of a group of welan gums and is produced byfermentation with a microorganism of the genus Alcaligenes. Anotherwelan gum heteropolysaccharide, known as S-194, is also produced byfermentation with a microorganism of the genus Alcaligenes. A notablecharacteristic of the heteropolysaccharide S-130 is that it develops ahigh viscosity in saline waters. This is particularly so in brines whichcontain divalent cations such as Ca²⁺ and M²⁺ or monovalent cations suchas Na⁺ and K⁺. U.S. Pat. No. 4,658,898 discloses the use of welan gumS-130 in saline waters. Crosslinking with trivalent cations, such aschromium, aluminum, zirconium and iron is also disclosed.

The use of various block copolymers for mobility control inwaterflooding operations is described in U.S. Pat. Nos. 4,110,232,4,120,801 and 4,222,881. Chung et al., U.S. Pat. No. 4,653,585, disclosethe use of block copolymers, which may be crosslinked with polyvalentmetal ions, for use as permeability control agents in enhanced oilrecovery applications.

While a number of the different compositions discussed have beenproposed for permeability control, some of these compositions may beunsuitable for use as permeability control agents under certaincircumstances. For example, the polymers of Chung et al, may not beeffectively crosslinked with polyvalent metal ions under all conditionsencountered in the enhanced oil recovery applications, e.g., in acidicconditions commonly found in carbon dioxide (CO₂) flooding operations.Polyacrylamides display instability in the presence of high brineconcentration at temperatures oven 70° C. Xanthan gums are very brinetolerant but display thermal instability, even at temperatures below 60°C. Still, other polymers are unsuitable as permeability control agentswhen used in conjunction with steam flooding operations. This is due tothe fact that they lose their structural integrity (i.e., they undergo"syneresis") at the high temperatures generated during such operations.

Syneresis is the contraction or shrinking of a gel so that liquid isexuded at the gel surface. For example, a gel said to exhibit 20%syneresis would take up 80% of its original volume, with the remaining20% being expelled water. Although the exact mechanism responsible forthe syneresis of such gel-forming compositions is not fully understood,it is believed to result from the over-crosslinking of the polymericmaterial with time. While it is not yet known what an acceptable levelof syneresis might be for profile control gels, it is believed that tominimize syneresis would enhance the effectiveness of such gels.

In view, therefore, of the severe conditions which include both highbrine concentrations, elevated temperatures, or both, there is a needfor brine tolerant, thermally-stable materials suitable for hightemperature wells and steam flooded walls. This has led to thedevelopment of the so-called hostile environment polymers, such as thosemarketed by the Phillips Petroleum Company of Bartlesville, Okla. andthe Hoechst Celanese Corporation of Somerville, N.J.

Despite these developments, a need still exsist for permeability controlagents which are compatible with the harsh conditions encountered insteam flooding or other enhanced oil recovery operations in which hightemperatures and/or conditions of high salinity are encountered.

It is, therefore, an object of the present invention to provide apolymer gel-forming composition for injection in a subterraneanreservoir which has good thermal stability at high brine concentration.

It is another object of the present invention to provide a polymer gelwhich can be used effectively as a permeability control agent under theextreme temperature conditions encountered in the steam flooding ofunderground formations.

It is a further object of the present invention to provide an aqueouscrosslinked polyvinyl amine copolymer gel for use in profile controlapplications.

Other objects and the several advantages of the resent invention willbecome apparent to those skilled in the art upon a reading of thespecification and the claims appended thereto.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an aqueous-basedpolymeric composition capable of forming a stable gel under harshsubterranean formation conditions which is comprised of water, awater-dispersible copolymer of polyvinyl amine and a crosslinking agentwhich is either a preformed phenolic resin or a mixture of a phenoliccomponent and an aldehyde. The gel-forming compostion is an effectivepermeability control agent and finds particular utility in steamflooding operations where it is stable even when conditions of highsalinity and high underground formation temperatures are encountered.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Any water-dispersible or water-soluble polyvinyl amine copolymer capableof forming stable aqueous gels in the presence of a phenolic resin orphenol/aldehyde mixture is contemplated for use in the practice of thepresent invention. Preferred are copolymers of vinylamine andvinylamide. Particularly preferred are the copolymers of vinylamine andvinylamide having the following structure: ##STR1## wherein: R may be analkyl or aryl group having no more than 16 carbo carbon atoms; and,

a and b are mole fractions of each co-monomer unit such that a+b=1, anda≠o and b≠o.

In the usual case, copolymers having a molecular weight of at leastabout 10,000, with about 2 mole percent vinylamine units, can be used toform advantageous gels. More desirable are copolymers having a molecularweight of about 100,000, with about 10 to about 30 mole percent ofvinylamine units. The vinylamide comonomer unit can be an N-vinylalkylor an N-arylamide such as N-vinylacarbamate, N-vinylacetamide orN-vinylphthalimide. Processes for the production of these copolymericmaterials are described in U.S. Pat. Nos. 4,795,770, 4,798,871 and4,804,793, the contents of each incorporated by reference in theirentirety for all that they disclose.

The crosslinking agents useful in the practice of this invention areselected from the group consisting of preformed phenolic resins andmixtures of a phenolic component and an aldehyde. Phenolic componentssuitable for use in the present invention are phenol or derivativesthereof, such as catechol, resorcinol, phloroglucinol, pyrogallol,4,4'-diphenol, and 1,3-dihydroxynaphthalen. Other phenolic componentsthat can be used include at least one member of selected oxidizedphenolic materials of natural or synthetic origin, such as1,4-benzoquinone, hydroquinone or quinhydrone, as well as a natural ofmodified tannin, such as quebracho or sulfomethylated quebrachopossessing a degree of sulfomethylation (DSM) up to about 50. (See U.S.Pat. No. 3,344,063, Col. 3, lines 15-32, which is hereby incorporatedherein by reference). The DSM of sulfomethylated quebracho (SMQ) issometimes indicated as, for example, SMQ 50 for SMQ having a DSM of 50.Phenol is the preferred phenolic compound for use in the presentinvention.

Any suitable water-dispersible aldehyde can be used in the practice ofthe invention. Thus, under proper conditions of use, both aliphatic andaromatic moinoaldehydes, and also dialdehydes, can be used. Thealiphatic monoaldehydes containing from one to about 10 carbon atoms permolecule are preferred. Representative examples of such aldehydesinclude formaldehyde, paraformaldehyde, acetaldehyde, propionaldehyde,butyraldehyde, isobutyraldehyde, valeraldehyde, heptaldehyde anddecanal. Representative examples of dialdehydes include glyoxal,glutaraldehyde and terephthaldehyde. Various mixtures of such aldehydescan also be used in the practice of the invention. The term"water-dispersible" is employed generically herein to include both,those aldehydes which are truly water-soluble and those aldehydes oflimited water solubility but which are dispersible in water or otheraqueous media to be effective gelling agents. Formaldehyde is thepreferred aldehyde compound used in the present invention.

Specific examples of suitable phenolic and water-dispersible aldehydecomponents are set forth in Swanson, U.S. Pat. No. 4,440,228, the entirecontents of which are incorporated herein by reference. The preferredcombination of a phenolic and water-dispersible aldehyde isphenol/formaldehyde.

Relative amounts of phenolic and aldehyde components are also set forthin Swanson. These amounts are small but effective to cause the gelationof an aqueous dispersion of the copolymr and the crosslinking agent.Thus, the amount of the phenolic component used herein is about 2 toabout 20, preferably about 2 to about 10, and most preferably about 3 toabout 6 percent by weight. Rigid and more thermally stable gels aregenerally formed at higher phenol concentrations. Aldehyde to phenolmolar ratios of between about 1 to 4 are preferred. As such, the amountof aldehyde is about 0.002 to about 15 weight percent, preferably about3 to about 10 and most preferably about 3 to about 7 weight percent. Theabove amounts are calculated on the basis of the total weight of thecomposition comprising the water, the copolymer and the crosslinkingagent.

The concentrations of polyvinyl amine copolymer necessary to prepare thegel-forming compositions of the present invention will generally in therange of from about 1.0 to about 20.0 weight percent in order to achievea useful gel consistency; in most cases, however, concentrations of 1.0to 10.0 weight percent will be adequate and normally preferred, althoughreservoir conditions may require other concentrations. Generally,stronger and more stable gels are formed at higher copolymerconcentrations. The molecular weight of the copolymer used will alsoaffect the amount of material required to form advantageous gels.Generally, the higher the molecular weight of a polyvinyl aminecopolymer, the lower the amount required to form a stable gel. As hasbeen previously stated, it is preferred that the copolymer have amolecular weight of at least about 10,000, with a molecular weight of atleast about 100,000 particularly preferred.

The gel-forming compositions of the invention can be prepared on thesurface, in a suitable tank equipped with suitable mixing means, andthen pumped down the well and into the formation employing conventionalequipment for pumping such compositions. Additionally, it is within thescope of the invention to prepare these compositions while they arebeing pumped down the well. For example, a solution of the copolymer inwater can be prepared in a tank adjacent to the wellhead. Pumping ofthis solution through a conduit to the wellhead can then be started.Then, downstream from the tank, a suitable connection can be providedfor introducing a crosslinking agent of this invention. As will beunderstood by those skilled in the art, the rate of introduction of thecrosslinking agents into the conduit will depend upon the pumping rateof the copolymer solution through the conduit. Any of theabove-mentioned orders of addition can be employed in such in situtechnique. Mixing orifices or baffles can be provided in the conduit, ifdesired. When gelation is to take place at lower temperatures (<300°F.), base catalysts such as NaOH, Ba(OH)₂, Ca(OH)₂, Na.sub. 2 CO₃ or thelike may be required to assist the reaction.

After the cmposition is injected into the formation and gels, the oilrecovery process is conducted in the usual manner, i.e., a displacingfluid, which is miscible or immiscible with the oil, is injected intothe formation and the oil is subsequently recovered in a conventionalmanner. Suitable oil-miscible displacing fluids are carbon dioxide(CO₂), carbon monoxide (CO), methane, ethane, propane, butane, naturalgas, liquid petroleum gas and mixture thereof. CO₂ is the preferredoil-miscible displacing fluid. Suitable oil-immiscible displacing fluidsare carbon dioxide, used under oil-immiscible conditions, water or anaqueous fluid, nitrogen, ambient air, steam, flue gas, and mixturesthereof. Because the gel-forming composition of the present invention issubstantially stable at high temperatures (up to 400° F.), and does notundergo significant syneresis at such temperatures, it is preferablyused in conjunction with the steam flooding of underground oilformations.

The gel-forming compositions of the present invention may also be usedin a so-called Water Alternating Gas (WAG) process, well known to thoseskilled in the art. In such a process, the injection of slugs of wateris alternated with the injection of slugs of gas, such as CO₂. If theWAG process is used with the compositions of the invention, suchcomposition or compositions are injected into the formation with one ormore water slugs.

After the miscible transition zone is established between the formationoil and the displacing fluid, a driving fluid may be injected throughthe injection well to displace the oil, the transition zone and thedisplacing fluid through the formation towards the production well fromwhich the oil is produced. The driving fluid is injected for asufficient time to effect the displacement of the formation oil to theproduction well until either all of the oil has been displaced from theformation or until the economical limit of the ratio of the drivingfluid to the formation oil has been reached.

The driving fluid (also referred to herein as a drive fluid) used in theprocess of the invention may be any fluid known to those skilled in theart as suitable for that purpose, but preferably it is a fluid selectedfrom the group consisting of water, brine, methane, carbon dioxide,nitrogen, air, steam, separator gas, natural gas, flue gas and mixturesthereof. The driving fluid may contain additives, such as a surfactant,to maintain efficient displacement thereof.

It is within the scope of the invention to precede the injection of thegel-forming composition into the well and out into the formation with apreflush of a suitable cooling fluid, e.g., water. Such fluids cool thewell tubing and formation and may extend the useful operatingtemperature range of the gelled composition. The volume of the coolingfluid injected into the well can be any suitable volume sufficient tosignificantly decrease the temperature of the formation being treated,and can vary depending upon the characteristics of the formation. Forexample, amounts of up to 20,000 gallons or more can be used to obtain atemperature decrease on the order of 100° to 250° F.

The copolymers used in the practice of this invention are dissolved inwater, at concentrations of between about 1.0 to about 20.0 weightpercent, preferably between about 1.0 to about 10.0 weight percent. Thesolution is then injected into the formation, so as to selectively blockthe more highly permeable regions, to control the subsequent floodingoperation which may be carried out in a conventional manner. Injectionof the solution into the formation may be carried out in a conventionalmanner using an injection well which extends from the surface of theearth into the formation, e.g. as described in U.S. Pat. Nos. 4,078,607,3,305,016, 4,076,074, 4,009,755 and 4,069,869, to which reference ismade for the descriptions of typical procedures which can be usedherein. Briefly, the aqueous gel-forming liquid is injected into theformation through the injection well. Once in place in the more highlypermeable regions, the gel controls subsequent flooding operations bydiverting the flood liquid, such as water or CO₂, to the less permeableor "tight" zones, increasing recovery from these zones. The amount ofthe viscous solution which is injected into the reservoir will generallybe from about 10% to about 100% of the pore volume of the highpermeability stratum or strata.

Because the compositions of the crosslinked copolymers may be readilyvaried, e.g., by changing the type or amount of the crosslinking agent,etc., the characteristics of the resultant gels may also be varied. Thisinvention therefore offers the possbility of formulating polymeric gelsaccording to specific reservoir conditions.

The following examples further illustrate the essential features of theinvention. However, it will be apparent to those skilled in the art thatthe specific conditions used in the examples do not limit the scope ofthe invention.

EXAMPLE 1-5

A poly (vinylamine-vinylamide) copolymer having about 15 mole percent ofvinylamine units and about 85 mole percent of vinylamide units, with amolecular weight of about 2,000,000 was used in the formation of thegels of these examples. The copolymer was obtained from Air Products andChemicals, Inc. of Allentown, Pa. Phenol and formaldehyde were used atvarious concentrations to form the crosslinking agent for the aqueoussolutions of the copolymeric material. NaOH was used in Examples 2 and 4to serve as a catalyst for the crosslinking reaction.

The compositoions of Examples 1-5 were prepared in a synthetic sea waterbrine. Thermal stability of those compositions were determine bysubjecting small samples thereof (about 10 gr) to a temperature of about400° F. for an extended time period. The samples, in small glas vials,were inserted into stainless steel bombs (1" O.D. tubing with Swagelockcaps), partially surrounded with water, sealed, placed in an oven, andmaintained at the temperature of about 400° F. for the length of timeindicted in Table 1. After the testing period, the degree of syneresisof each sample was determined by cooling the bomb to ambienttemperature, removing the vial from the bomb, estimating the size of thepolymer plug by measuring the height and diameter of the gel plug, andcalculating the degree of syneresis (%) from the following equation:##EQU1## wherein: "Sample Volume" is the initial volume of the samplebefore the heating treatment is commenced; and

"Gel Volume" is the final volume of the gel present in the vial afterthe heating treatment is terminated.

Results of the thermal stability tests are as follows:

                  TABLE 1                                                         ______________________________________                                        Poly (vinylamine-vinylamide) Gel Test                                                                     Days    Gel                                       Example                                                                              Composition, %       @       Syneresis                                 Number Polymer  Phenol  HCHO  NaOH  400° F.                                                                      %                                   ______________________________________                                        1      2.0      8       8.4   0.0   24    0                                   2      2.0      4       4.2   0.4   24    0                                   3      2.0      4       4.2   0.0   24    0                                   4      2.0      4       5.1   0.4   24    0                                   5      2.0      3       3.9   0.0   24    0                                   ______________________________________                                    

As indicated, gels of excellent thermal stability were obtained in allcases.

As may be seen from these test results, all gels were found to be verystable and would be expected to perform well even under the harshconditions encountered during steam flooding enhanced oil recoveryoperations.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be utilized without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchmodifications and variations are considered to be within the purview andscope of the appended claims.

What is claimed is:
 1. A process for recovering oil from a subterraneanoil-bearing formation having relatively high permeability zones andrelatively low permeability zones penetrated by an injection well and atleast one production well in fluid communication with a substantialportion of the formation, comprising the steps of:(a) injecting into therelatively high permeability zones of the formation via the injectionwell an aqueous gel-forming composition comprising water, a viscosifyingamount of a water-dispersible polyvinyl amine copolymer; and acrosslinking agent which is a mixture of a phenolic component and analdehyde, said crosslinking agent present in an amount effective tocause gelation of said polyvinyl amine copolymer; (b) injecting aflooding fluid into the formation via the injection well thatpreferentially enters the low permeability zones; and (c) recoveringfluids including oil from the formation via the production well.
 2. Theprocess of claim 1, wherein in step (a) said polyvinyl amine copolymeris a copolymer of vinylamine and vinylamide having the structure:##STR2## wherein: R is an alkyl or aryl group having no more than 16carbon atoms; anda and b are mole fractions of each comonomer unit suchthat a+b=1, and a≠o and b≠o.
 3. The process of claim 2 wherein in step(a) said polyvinyl amines copolymer has a molecular weight of at leastabout 10,000 and the vinylamine comonomer is present in an amount of atleast about 2 mole percent.
 4. The process of claim 3, wherein in step(a) said polyvinyl amine copolymer has a molecular weight of at leastabout 100,000 and the vinylamine comonomer is present in an amount ofabout 0 to 30 mole percent.
 5. The process of claim 3, wherein in step(a) said vinylamide comonomer is selected from the group consisting ofN-vinylalkyls and N-arylamides.
 6. The process of claim 5, wherein insaid step (a) said phenolic mixture comprise about 1 to 99 weightpercent of at least one phenolic compound selected from the groupconsisting of phenol, resorcinol, catechol, phloroglucinol, pyrogallol,4,4'-diphenol and 1,3-dihydroxynaphthaline; and about 1 to about 99weight percent of at least one aldehyde component selected from thegroup consisting of aliphatic monoaldehydes, aromatic monoaldehydes,aliphatic dialdehydes and aromatic dialdehydes.
 7. The process of claim6, wherein in step (a) said phenolic compound in phenol and saidaldehyde component is formaldehyde.
 8. The process of claim 7, whereinin step (a) said phenol and said formaldehyde are present in a molarratio of about 1 to
 4. 9. The process of claim 8, wherein in step (a)said polyvinyl copolymer is present in an amount of between about 1 and10 weight percent.
 10. The process of claim 1, wherein the floodingfluid of step (b) is steam.